1. Field of the Invention
The invention relates to apparatus for gas and liquid chromatography. More particularly, the invention relates to apparatus in which the chromatographic columns are heated by microwave.
2. Description of the Prior Art
Gas and liquid chromatography are physical methods for the separation and identification of chemical compounds. Briefly, gas chromatography is a method in which the components of a mixture are separated from one another by introducing a sample of the mixture into a carrier gas stream which percolates through or along a stationary bed of large surface area. The stationary bed is normally contained within a chromatographic column. The different components of the sample move through the column at different rates and emerge one after the other at the outlet of the column where they are detected and measured. A typical gas chromatograph apparatus includes a chromatographic column, a carrier gas, means for establishing a flow of carrier gas through the column, means for introducing a sample of the mixture to be analyzed into the carrier gas prior to its entry into the column, and a detecting device which is capable of indicating the presence of the various components of the sample both qualitatively and quantitatively as they emerge from the column. All of this is described in greater detail in "Gas Chromatography," by Keulemans, Reinhold, New York (1959), the contents of which are incorporated by reference.
Liquid chromatography is based on separation of the components of a mixture in solution by selective adsorption, and, in some aspects, is similar to gas chromatography except that in liquid chromatography a solvent medium is used instead of the carrier gas utilized in gas chromatography. A typical liquid chromatograph includes a moving solvent, a means for producing solvent motion (i.e., a pump), means for introducing a sample into the solvent prior to its entry into the column, a chromatographic column and a detector. All of this is described in greater detail in "Modern Liquid Chromatography" by Snyder and Kirkland, Wiley Interscience (1974), the contents of which are incorporated by reference.
Both liquid and gas chromatography are widely used for analytical determinations. For example, in many industrial processes, chromatography can be used to detect the presence of impurities in the products obtained at one or more stages of manufacture. Also, boiling range analysis of many products may be obtained quickly and simply by chromatograph. In addition, component-by-component analysis of many materials may be effected by chromatography.
In many of the analysis discussed above, the separation of the components of the sample mixture is enhanced by the application of heat to the chromatographic column. In some analysis, such as boiling point analysis, the rapid and precise application of heat is critical to the accuracy of the analysis.
Prior to the present invention, chromatographic columns were heated via electric resistance elements or ovens. For example, U.S. Pat. No. 3,527,567 of Philyaw describes a gas chromatograph apparatus in which the chromatographic column is heated by placing it in an electric oven.
This method of applying heat to the chromatographic column, however, suffers from serious disadvantages. For example, precise temperatures control over the entire length of the column is different to obtain. In addition, the time it takes to heat the column to the desired temperature may be extremely long, thus limiting the number of analysis that can be conducted on a given piece of equipment during a given time. Finally, in situations where the temperature of the columns are raised throughout the analysis, i.e., the columns are "temperature programmed", the time it takes to cool the oven and columns back to the initial temperature so that another run can be started may be substantial.
In order to alleviate the difficulties outlined above and to provide rapid and substantially uniform heating and cooling of the chromatographic columns, Burow et al., in U.S. Pat. No. 3,232,093, placed the chromatographic column adjacent to a second column which could be cooled by means of a heat exchange fluid. Heat was supplied by wrapping both columns with electrical resistance wire. In U.S. Pat. No. 3,169,389, Green et al. provide a system for the heating of the columns of a gas chromatograph in which the column itself acts as an electrical resistance heating element. Neither of these patents disclose the microwave means for heating chromatograph columns of the present invention.